The induction heat treating process is used in various applications for hardening, and annealing of metals. The process includes applying energy directly to metals and other conductive materials via an alternating electric current passing through an induction heating coil positioned in close proximity to a workpiece. The induction heating process is applicable to both continuous and component heating. Continuous heating relates to processes such as wire and strip manufacturing and includes induction heating coils used for forging products, billet heaters and tube annealing. Component heating is describes a process for heating one component, or workpiece at a time. Gears and axle shafts are generally hardened by component induction heating processes.
A common use for component induction heating is case hardening of carbon steel, or alloy parts for use in the formation of automobiles, farm equipment, airplanes and other production apparatuses. Component induction heating rapidly heats the workpiece in a short period of time. The workpiece is then quenched and a hardened surface, or through hardened part is formed. The depth of the hardened surface is regulated by the frequency of current, temperature of the part surface, and quenching of the part.
Additionally, induction heating coils may be used to continuously heat a workpiece, or billet, prior to stamping or other fabrication process. The billet is heated gradually to a desired temperature by passing through an extended induction heating coil or a series of induction heating coils of increasing temperature. The series of induction heating coils, or alternatively a single coil, will heat the billet by maintaining energization over the duration of the continuous heating process. More specifically, a helically wound induction heating coil or a series of coils may be used to continuously heat a billet prior to stamping. Each induction heating coil, or series of coils, is encased in refractory material to help retain the heat necessary to pre-heat a billet and act as a heat shield to protect the coil from excess heat. This continuous process may require an induction heating coil to be energized for extended time periods of 2000–4000 hours, or a lifespan of approximately one month.
Much of the prior art is directed to systems for measuring and maintaining the temper and surface hardness to insure proper performance and quality control of the heated parts. The concept of monitoring an induction heating cycle is disclosed in U.S. Pat. Nos. 4,897,518 and 4,816,633 to Mucha et al. and for monitoring the current in an induction heating coil is disclosed in U.S. Pat. No. 5,434,389 to Griebel. These prior patents are incorporated by reference herein for general background information as they relate to the conventional induction heating treating processes. Similarly, U.S. Pat. Nos. 3,746,825 and 5,250,776 to Pfaffmann disclose a method for measuring input energy and temperature and heating rate of a workpiece, respectively. U.S. Pat. No. 6,455,825 to Bentley et al. discloses the use of miniature magnetic sensors strategically placed about the workpiece to monitor changes in the magnetic properties of the workpiece as it heats up during induction heating and cools down during quenching. These patents are also incorporated by reference for the further purpose of illustrating the state of the art of induction monitoring systems.
Both conventional induction heat treating processes are detrimental to the perishable heat treating tool. The tool, or inductor coil, is designed and shaped specifically to the workpiece undergoing the heat treatment. An induction heating machine may include a specifically designed coil, or multiple identical coils mounted to the machine, or various coil designs mounted to a single machine in series, all used for heating or hardening various workpieces during production. Each coil may be formed of multiple copper parts and flux concentrators that are brazed or attached to form an inductor assembly. The joints have a limited life cycle and are prone to failure or leakage and must be repaired. Further, arcing often occurs where there are small air gaps between the tool and the workpiece causing stress cracks and damage to the coil. During continuous heat induction, the surrounding refractory material tends to breakdown due to the heat or other property failures. These examples only exacerbate the already short tooling life of a coil and lead to costly repairs. Each time tooling is changed, the induction heating machine and the heat treated parts must be validated to ensure that the new coil is performing per required specifications. Tooling and production shutdown are costly and time-consuming. Employing multiple coils with each machine, without knowing the cycle history of each individual coil increases the opportunity for production interruption.
Currently, an end user/purchaser of induction heating equipment will contract an induction equipment supplier (OEM) to design an optimal coil configuration for the part requiring induction heating. Based on the quality of material used and quality of workmanship, the coil will need repairing after an unknown amount of cycles or duration of energization. More often than not, the end user will choose to send the coil to an after market company for the repair based mainly on the cost of the repair. A costly inventory of inductor coils is maintained at the production site for immediate replacement when a coil fails during production. Occasionally a replacement coil is removed from inventory without ordering new replacements, thus creating an immediate need for a new replacement coil.
A blind count is recorded of how many times the induction heating machine is cycled for purposes of determining the amount of parts that have been heat treated. However, no record is kept of how many times each individual inductor coil is energized, or cycled, or the duration of energization of the coil during a heating process. Nor is a record kept of how many different inductor coils are used in a multiple coil machine. Therefore, no hard record is created to determine the cycle life of each inductor coil, i.e. how many cumulative cycles in the life of an average inductor coil or the duration of time the coil has been energized for heating a workpiece. Best estimates are that a perishable coil must be replaced approximately every 5,000 to 100,000 cycles based on each individual application or every 2,000 to 4,000 hours of prolonged energization. These tool costs are incorporated into the overall cost of each manufactured part.
When an inductor coil fails, production stops. The coil must be changed and the machine and subsequently heat treated parts must be validated. This requires the transportation and quarantine of the parts to a separate storage area for analysis of quality control. If the parts do not meet the specified criteria, they are scrapped, resulting in an expensive waste of material and labor. The alternative option is to wait until the metallurgical results are verified before running production, this may take hours.